This invention relates to a control system for polyphase electrical motors. More particularly, it relates to an electronic system for providing sinusoidal excitation signals to ac and brushless dc motors.
Traditionally, dc electric motors utilized a brush and commutator design including a rotor located within a stator. In a dc motor, a rotating field-flux is established in the air gap between the rotor and stator by the stator, either through permanent magnets or by a field winding distributed about the stator. Generally, the rotor has a plurality of windings, distributed about its axis, which accomodate the current required to achieve the desired electrical power level. The rotor is in turn coupled to the motor shaft. The brush and commutator mechanism switches excitation currents from a dc power source to the rotor windings. The rotor revolves as a result of the interaction between the field-flux created in the air gap and the current flowing in the rotor windings.
It is well known in the art that the torque contribution by any one rotor winding varies sinusoidally with the rotor position. To establish continuous torque, the brush-type motors typically employ many separate rotor windings. As a result, by the time that the torque contribution due to one rotor winding is detectably waning due to a change in the rotor position, the brush and commutator mechanism is already switching the excitation current to the next rotor winding. Consequently, brush-type motors provide a relatively constant shaft torque.
Nevertheless, brush-type motors suffer from several reliability problems. One problem is that the brushes, typically made of carbon, wear away, creating a conductive carbon powder which must be cleaned out periodically. Another problem is that sparking, at the commutator, makes it difficult to use the motor in many environments. A third problem is that periodic replacement of the brushes and re-machining of the commutator segments is necessary.
An alternate to a dc motor is an induction motor. Such motors include power handling or primary windings on the stator, with secondary windings arranged in a shorted turn configuration about the rotor. Through the use of the shorted turn configuration, induction motors avoid the need for mechanical brushes and commutators. As a result, induction motors are generally considered more reliable and less expensive to maintain then their brushed predecessors. Consequently, they are popular as replacements for the older brush-type designs. However, induction motors are considered somewhat more difficult to control, particularly in variable speed applications and in servo systems which require many velocity changes and torque reversals. Such is a result of the nonlinear relationship between torque and applied current in an induction motor.
In the last ten or fifteen years, a new class of motor, the brushless dc or synchronous motor, has been developed. It combines the control ease of the brush-type motor with the reliability of the induction motor. Field flux is typically established by a permanent magnet assembly contained in the rotor. An excitation current is periodically switched between a plurality of phase windings distributed about the stator. Torque is generated, and the motor shaft rotates, as a result of the interaction between the field-flux in the air gap and the phase winding currents.
Instead of switching phase winding currents with a brush and commutator mechanism, synchronous motors generally utilize external electronic switches. A typical controller senses shaft position (e.g., using a resolver or magnetic reed switches), and then uses the position information to control switching (typically with transistors) of excitation currents to particular phase windings at the appropriate times.
Unlike the brush-type dc motors, the newer motors usually have only three phase windings, requiring six commutation changes per cycle. Early brushless motor controllers, called "six-step" controllers, utilized whatever drive voltage was available and abruptly switched it between the phase windings in a periodic six-step sequence. Such an approach tended to produce substantial torque ripple and mechanical noise.
One solution to the torque ripple problem has been to employ what are known as sinewave controllers. In a sinewave controller, the rotor windings are driven with a voltage (or current) excitation which smoothly transits from winding to winding instead of being abruptly switched. This is accomplished in some prior art three phase systems by using a plurality of high frequency, two-state modulators driving electronic switches to synthesize two drive current phases from a dc power source. One phase current is coupled into a first stator winding while the other is coupled into a second stator winding. The current in the third stator winding is necessarily set by Kirchoff's Current Law, as the negative of the sum of the currents in the first and second windings. Therefore, the third winding is typically driven with a voltage drive. One such system is disclosed in U.S. Pat. Nos. 4,306,182 and 4,467,262. According to those patents, the voltage, coupled to the third winding is controlled as a function of the current error signals from the other two terminals.
While sinewave drive controllers such as disclosed in U.S. Pat. Nos. 4,306,182 and 4,467,262, provide a quiet motor drive system with low torque ripple, they generate less power and must operate at lower speed than do their square wave counterparts. The reason is that a square wave of a particular frequency contains more energy than does a sinewave of the same peak amplitude and same fundamental frequency.
Accordingly, one object of the present invention is to provide a system which minimizes the effect of the power loss suffered as a result of utilizing a sinewave type motor controller.
A second object of the invention is to maximize the power transferred to a particular motor from a particular dc voltage source by enhancing the dynamic range available to the motor terminal voltages and thus, the excitation signals applied to the motor's primary windings.
Another object of the invention is to provide a system for modulating a voltage source to set the voltage at the wye-center node in a way which maximizes the dynamic range available to the motor terminal voltages.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.